| FRA-7700 and FRT-7700 Modifications (FRA-7700: From Active Antenna to Preselector) |
Don't let my scratched out notes scare you. Take some time to see what I did here. First, note the little block diagram at the top with
the 'filter' and the 'amp'. That's the concept; filter first, then amplify or attenuate as needed.
Then look at the 2 DPDT push button switches which have been completely disconnected and rewired as shown. Follow the
insertion points 'A' thru 'E' and add the connectors of your choice. There's some 'land cutting' on the PC Bd. and some jumpers to
add, but following the notes, one-by-one, should get you through without problems.
I've modified 2 of these and they are really great assets to a complete listening post. The first one took a full Saturday afternoon
because I had to trace things out and design as I went along. The second one took about 2 hours.
the 'filter' and the 'amp'. That's the concept; filter first, then amplify or attenuate as needed.
Then look at the 2 DPDT push button switches which have been completely disconnected and rewired as shown. Follow the
insertion points 'A' thru 'E' and add the connectors of your choice. There's some 'land cutting' on the PC Bd. and some jumpers to
add, but following the notes, one-by-one, should get you through without problems.
I've modified 2 of these and they are really great assets to a complete listening post. The first one took a full Saturday afternoon
because I had to trace things out and design as I went along. The second one took about 2 hours.
The tuner mods. Note that the "Antenna" switch is now an "ANT or GND" switch and the "Out" switch is now an "Attenuator Bypass"
switch. Study the schematic. There really isn't too much to do on this one. I suggest that you print this page for study purposes. Also,
right-click the schematics and save them to a folder. Later, you can make blow-ups and print those to study as well.
Perform these modifications at your own risk. Changing, modifying, or altering any product, in any way, may invalidate any warranties,
and/or cause refusal of repair. These data are presented here for informational and archival purposes only and do not guarantee any
specific performance or operational standards.
The author cannot be responsible for others' craftsmanship or workmanship.
Never work on "LIVE" circuitry.
Always disconnect/unplug all equipment before opening the case or attempting to work on internal circuitry.
switch. Study the schematic. There really isn't too much to do on this one. I suggest that you print this page for study purposes. Also,
right-click the schematics and save them to a folder. Later, you can make blow-ups and print those to study as well.
Perform these modifications at your own risk. Changing, modifying, or altering any product, in any way, may invalidate any warranties,
and/or cause refusal of repair. These data are presented here for informational and archival purposes only and do not guarantee any
specific performance or operational standards.
The author cannot be responsible for others' craftsmanship or workmanship.
Never work on "LIVE" circuitry.
Always disconnect/unplug all equipment before opening the case or attempting to work on internal circuitry.
A Single Antenna Port
for the FRG-7700
Condition: The FRG-7700 comes with two
antenna ports. One is a low impedance input
for a shortwave (HF) dipole, transformer
(balun/unun) fed random wire, or similar
antenna. The other is for a high impedance
AM (MW) broadcast antenna. This is fine if
you use the FRT-7700 tuner or FRA-7700
active antenna, but if you don't have, or use,
those devices, or if you have completed the
modifications on the previous two pages,
then the two ports are an inconvenience.
Analysis: Feeding the SW port only, causes a
36 dB loss when you tune the BC band.
Feeding just the BC input gives you virtually
no signal for the SW bands. I tried several
design approaches in an attempt to "unite"
these two inputs. On the test bench, several
measurements were made in each
condition, on each band, with every
permutation possible. The final verdict was
boiled down to a simple option of mixing the
two input filter sections with, either (A) a fixed
resistor (50 to 200 Ohms)* or (B) a variable
resistor (20 K Ohm potentiometer).
The higher value of separation offered by the
pot allows the front end to give max. gain.
When listening between SW and AM
Broadcast, just adjust the pot for max
S-meter readings. Using the fixed resistor
will spoil the sensitivity by anywhere from 2 to
4 dB. Since I already have electro-OCD, I
decided to drill the chassis and go for the full
flexibility by using a "mixing pot."
*(Use a carbon composition or carbon film
type, not a wirewound resistor)
for the FRG-7700
Condition: The FRG-7700 comes with two
antenna ports. One is a low impedance input
for a shortwave (HF) dipole, transformer
(balun/unun) fed random wire, or similar
antenna. The other is for a high impedance
AM (MW) broadcast antenna. This is fine if
you use the FRT-7700 tuner or FRA-7700
active antenna, but if you don't have, or use,
those devices, or if you have completed the
modifications on the previous two pages,
then the two ports are an inconvenience.
Analysis: Feeding the SW port only, causes a
36 dB loss when you tune the BC band.
Feeding just the BC input gives you virtually
no signal for the SW bands. I tried several
design approaches in an attempt to "unite"
these two inputs. On the test bench, several
measurements were made in each
condition, on each band, with every
permutation possible. The final verdict was
boiled down to a simple option of mixing the
two input filter sections with, either (A) a fixed
resistor (50 to 200 Ohms)* or (B) a variable
resistor (20 K Ohm potentiometer).
The higher value of separation offered by the
pot allows the front end to give max. gain.
When listening between SW and AM
Broadcast, just adjust the pot for max
S-meter readings. Using the fixed resistor
will spoil the sensitivity by anywhere from 2 to
4 dB. Since I already have electro-OCD, I
decided to drill the chassis and go for the full
flexibility by using a "mixing pot."
*(Use a carbon composition or carbon film
type, not a wirewound resistor)
Above is a view of the 20 K Ohm pot and the two wires which connect it to the two
antenna ports. Note that the SO-239 connector is already connected to the SW
circuitry.
Operation: The advantage of the pot over the fixed resistor is that you may adjust it for
either maximum signal on BC or for minimum loading and noise on SW. The
differences are minescule and barely noticeable under most conditions, but they are
visible on the test bench and can be heard under some extreme listening conditions.
Turning the pot to zero ohms, which shorts the two inputs together, provides
maximum signal transfer for BC, but may add a small amount of (mismatch) noise
while tuning SW.
Conversely, increasing the pot's resistance to, say, 10 or 15 K Ohms, isolates the
SW filter section of the RF amplifier stage, and provides the cleanest signal, but, as
mentioned above, will cause a significant signal attenuation on AM (MW), B'cast.
The fixed resistor mod is a good compromise, and the less-than-fanatic user will
probably never notice any deficiency, nor desire anything more in performance.
You can easily try the resistor route first, by simply inserting it into both ports. Listen
to all the bands with the resistor in, with the ports shorted, and with both ports
opened. This is easily accomplished with a couple of clip leads connected to the two
ports and quickly trying all three conditions. It is my feeling that, your antenna, a
particular station's signal strength, and/or your location, will dictate whether you
make the simple mod or the variable one.
Remember all the usual safety disclaimer stuff: Unplug the radio before opening it,
or working on it, and don't do any mods if you are not comfortable working on such
equipment, etc, etc ...
... use common sense, and remember to unplug your soldering iron when you are
done!
Happy listening.
antenna ports. Note that the SO-239 connector is already connected to the SW
circuitry.
Operation: The advantage of the pot over the fixed resistor is that you may adjust it for
either maximum signal on BC or for minimum loading and noise on SW. The
differences are minescule and barely noticeable under most conditions, but they are
visible on the test bench and can be heard under some extreme listening conditions.
Turning the pot to zero ohms, which shorts the two inputs together, provides
maximum signal transfer for BC, but may add a small amount of (mismatch) noise
while tuning SW.
Conversely, increasing the pot's resistance to, say, 10 or 15 K Ohms, isolates the
SW filter section of the RF amplifier stage, and provides the cleanest signal, but, as
mentioned above, will cause a significant signal attenuation on AM (MW), B'cast.
The fixed resistor mod is a good compromise, and the less-than-fanatic user will
probably never notice any deficiency, nor desire anything more in performance.
You can easily try the resistor route first, by simply inserting it into both ports. Listen
to all the bands with the resistor in, with the ports shorted, and with both ports
opened. This is easily accomplished with a couple of clip leads connected to the two
ports and quickly trying all three conditions. It is my feeling that, your antenna, a
particular station's signal strength, and/or your location, will dictate whether you
make the simple mod or the variable one.
Remember all the usual safety disclaimer stuff: Unplug the radio before opening it,
or working on it, and don't do any mods if you are not comfortable working on such
equipment, etc, etc ...
... use common sense, and remember to unplug your soldering iron when you are
done!
Happy listening.
| Unify the Two (rather confusing) Antenna Ports On the Yaesu FRG-7700 |
